河西走廊花海剖面晚冰期以来年代学及沉积特征研究

The Chronology and Characteristics of Sediments Since Late Glacial in Huahai Lake,Hexi Corridor,NW China

选择位于河西走廊的花海古湖泊沉积剖面作为研究对象,根据13个普通14C和5个AMS14C年代结果,以沉积物岩性特征为主要指标,建立了晚冰期以来花海湖泊沉积的年代框架及环境变化过程。结果表明:花海地区新仙女木期和晚冰期花海湖泊主要以芒硝沉积为主,指示了较低的温度环境。芒硝沉积中的淤泥细线为短暂升温标志,芒硝-淤泥-芒硝的沉积韵律揭示了晚冰期和新仙女木期气候的不稳定性和温度的周期性变化。根据沉积过程和岩性特征,全新世花海湖泊在千年尺度上存在干湿变化特征。10.47cal ka BP之前的早全新世气候较为干旱,以冲洪积和风成砂为主的沉积过程,10.47～8.87cal ka BP的早全新世湖相沉积所揭示的气候由干到湿的转变期,8.87～5.5cal ka BP存在深湖相沉积的气候湿润期,5.5cal ka BP至今沉积间断所揭示的中晚全新世气候呈现干旱化趋势,湖泊趋于萎缩干涸。

From the profile which is located in the Hexi Corridor, NW China. 13 samples were collected for conventional ^14C dating and 5 samples were collected for AMS ^14C dating. The materials used for ^14C dating were seeds, mud and wood. Based on the dating results, we reconstructed the age framework of Huahai Lake since the Late Glacial period. And then, we concluded that the sediments between 0.73 ～ 6.83 m were deposited during Holocene. Between 6.83 ～ 7.35 m, the sediments were deposited during the early Holocene and the late Younger Dryas. The sediments of 7.35 ～ 10.44 m were deposited during the Late Glacial and early Younger Dryas. Based on the mineral analysis of the sediments during the Younger Dryas and late glacial, mirabilite was the main component in the sediments, indicating the low-temperature environment. According to the chemical composition analysis , the main ingredient of the pure mirabilite deposition is Na2SO4 · 10H20,which is an indicator of the cold-dry environment; and the lower the tem- perature is good for the mineral crystallizing processes. The solubility curve of SO4^2- in Na2SO4-NaC1-H20 indicated that, in low surrounding temperature, the concentration of SO~ was lower in saturation Na2SO4, and simply increasing the lake level could not improve the ability of mirabilite solution, which was not powerful enough to break off the deposition. The main reason of the interruption for mirabilite deposition may be transient warming events. Therefore, the rhythm deposition formation of mirabilite-sih-mirabilite indicated the periodic changes of temperature during the late glacial and the Younger Dryas. According to the characteristics of the sediments and litbology, we reconstructed the millennium-scale effective moisture conditions during the Holocene period. Before 10.47 cal ka BP, the sediments color appears dark brown, purple brown-based, indicating a typical oxidizing environment, and this oxidative environment can not appear in the deep lake environment. There are no obvious lacustrine depositions layers at the beginning of Holocene（ before10.4 cal ka BP） , 6.30 ～ 3.73 m, and instead the layers are formed mainly on alluvial and eolian depositions, while the eolian depositions layers sandwiched between two alluvial depositions layers. The climate was relatively arid, and the alluvial and aeolian sediments prevailed during this period. Between 10.47 cal ka BP and 8.87 cal ka BP, the climate changed from arid to humid. During 8.87 - 5.5 cal ka BP the lake was relatively deep, and the climate was relatively humid. From 5.5 cal ka BP, there was little sediment in the section, showing that the lake became to dry up since then. This Holocene environmental change in millennium-scale was different from the westerlies in the arid Central Asian areas, but it was not the same as the environmental change in the eastern part of China, which was controlled by Asian monsoon. So the Holocene environment of the Huahai Lake region was affected by the Westerlies and the East Asian monsoon.